During semiconductor processing, plasma is often utilized to assist etching processes by facilitating the anisotropic removal of material along fine lines or within vias or contacts patterned on a semiconductor substrate. Examples of such plasma assisted etching include, for example, reactive ion etching (“RIE”), which is in essence an ion activated chemical etching process.
Although RIE has been in use for decades, its maturity is accompanied by several negative features, including: (a) broad ion energy distribution (“IED”); (b) various charge-induced side effects; and (c) feature-shape loading effects (i.e., micro-loading). Particularly, a broad IED contains ions having either too little, or too much, energy to be useful. Further, overly energetic ions are susceptible to causing semiconductor substrate damage. Additionally, broad IED makes it difficult to selectively activate a desired chemical reactions, where side reactions are often triggered by ions of sub-optimal, undesired energy.
Further, and over the course of the etching process, positive charge buildup on the semiconductor substrate may occur and repels ions that would otherwise be incident onto the semiconductor substrate. Alternatively, the charge buildup may produce local charge differences that affect currents on the substrate surface. Charge buildup may be due, in part, to the RF energy used to produce a negative bias on an otherwise non-conductive substrate used to attract positive ions from the plasma. Such RF frequencies are typically too high to allow a positive potential or near-neutral potential to exist for a sufficient amount of time to attract electrons for neutralizing the accumulated positive charges. Still further, non-uniform accumulation of charge across the surface of the substrate may create potential differences that may lead to currents on the semiconductor substrate that may very well damage the devices being formed.
One known, conventional approach to addressing these problems has been to utilize neutral beam processing. A true neutral beam process takes place essentially without any neutral thermal species participating as the reactant, additive, and/or etchant. Instead, the process at the substrate is activated by the kinetic energy of incident, directional, and energetic neutral species.
While neutral beam processes are not affected by flux-angle variation associated with the thermal species as in RIE, the use of neutrals has lead to absence of micro-loading efficiency. This lack of micro-loading results in a maximum etching efficiency of unity, in which one incident neutral nominally prompts only one etching reaction. Comparatively, in a RIE process the abundant thermal neutral etchant species may all participate in etching so that activation by one energetic incident ion may achieve an etch efficiency of 10, 100, and even 1000.
In plasma processing of substrates, particularly in chemical etching systems and other etching systems but also in some deposition systems, electro-negative gas is added to the processing gas. But the desired beneficial effects for which the electro-negative gas is used have been difficult to fully realize. Such desired beneficial effects include the production of appropriately energetic electrons, the production of negative ions and neutral species of the electro-negative gas, and the enhancement of fast positive ions for treatment of the substrate and for the activation of chemical reactions for processing the substrate.
For processing substrates having current high aspect ratio devices, generally a high energy flux of electrons, e.g., greater than 30 eV, is necessary. This energy level, and higher, reduce differential charging and minimize shading effects of the devices. Furthermore, the processing system must achieve high density plasmas (ne) while controlling polymerization. While high RF sheath voltage is needed for creating and delivering energetic ions to the substrate, the RF sheath voltage must be sufficiently low so that energetic electrons may be dumped onto the wafer surface.
Thus, there remains a need for a system that is configured to generate an ultra high density plasma without losing energetic electrons to trapping so as to dump the energetic electrons as the electrons are produced.